Adds 'ember:custom' include to portkey.json.
Adds 'ember:custom' include to portkey.json.

- This is needed to pull in the ember extension's code.
- 'custom' does not bring in file patterns from portkey.json
  but includes syntax highlighting and the `cname` and `mname`
  filters for use in templates.
- without this only the core portkey feature set would be available.

mutewinter/tapas-with-ember